The invention relates to a fuel injection system for internal combustion engines.
In a known fuel injection system of this type (DE-OS 29 42 010), a magnetic valve is arranged in the first relief duct as an open/close control device, and there is a control throttle in the second relief duct whose corss-section can be changed by a servomotor. The control throttle cooperates with an electronic control device in such a way that the servomotor is controlled by the electronic control device. While the start and end of delivery is determined by the magnetic valve which blocks the first relief duct or reopens it, they can also be effected in principle by other means, e.g. a mechanical open/close controlling means. The control of an additional flow-off quantity down to 0 quantity is effected by an adjustable throttle which enables a flow-off when the magnetic valve is closed and the injection takes place. This known system relates to a special modification of a process for achieving a quiet running of an engine by lengthening the injection period, this process being known in many different forms. During a partial quantity flow-off during the injection process, this flow-off quantity is compensated for by a corresponding lengthening of the injection period, for which purpose the effective displacement during a stroke of the pump plunger required for the injection is increased. The fact that an improvement of the quiet running is striven for particularly at lower engine speeds conflicts with this type of time-oriented control. In this instance however, the control of fuel quantity for maintaining speed is effected by using the engine speed as a control variable for determining the injection period, while the control throttle cross-section is determined by a presetting parameter corresponding to a predetermined quiet running effect. Thus, in this known fuel injection system, this "interference" with the fuel injection law effected by the adjustable throttle, the fuel injection law, per se, being controlled by the magnetic valve and determined by the electronic control device, is eliminated by changing the closing and opening times, respectively, of the magnetic valve by the electronic control device, so that the different flow-off quantity (interference variable) effected by the adjustable throttle depending on the throttle position, is compensated for. To this extent, this is a matter of a fuel quantity control with the speed as a control variable. However, such a control has the disadvantage that it is relatively sluggish, since the injection quantity is first changed by comparison of actual and desired values in the electronic control device before an additional correction is made possible by a new measurement. But actually this is only a matter of the "interference variable quantity" flowing off via the relief ducts which is to be compensated for.
Of course, it would also be possible to obtain the return flow quantity from the respective regulating variable of the servomotor actuating the adjustable throttle. However, in such compensation claculation, which is effected on the basis of response variables of the servomotor and is carried out in the electronic control device, complicated mathematical functions are involved, since the regulating distance of the servomotor has a correspondingly complicated relation to the return flow quantity flowing through the throttle, which return flow quantity would actually have to be calculated. This is a matter of functions of a higher degree, since pressure changes also occur because of the change in the throttle cross section and in connection with the dynamic relations during the injection processes.
This problem is solved in part in another known injection system (DE-PS 31 47 467) in that a magnetic valve is arranged tn two relief ducts in each instance, and a constant throttle is provided upstream of one magnetic valve, but with the restriction that this is a matter of relief ducts of the injection nozzle itself. The drawback of such a device with an electronic control system consists in that a separate arrangement is required for each injection nozzle, wherein, of course, as a result of the constant throttling during the interference quantity compensation, only the quadratic function between pressure and quantity of a throttling flow need be processed by the electronic control device. In order to be able to work with such a control, however, the start of injection must then be measured at every injection nozzle by an extra sensor, which results in additional expenditure and complication of the system.
Naturally, typical injection control processes not mentioned here are also burdened by these complicated regulating processes, such as that of the adjustment of the start of injection adapted to speed and load or the approximation of the injection quantity which, in particular, is a function of the speed and load.